The word "number" is also used in linguistics to describe the distinction between certain grammatical aspects that indicate the number of times an event occurs, such as the semelfactive aspect, the iterative aspect, etc. For that use of the term, see "Grammatical aspect".
Most languages of the world have formal means to express differences of number. One widespread distinction, found in English and many other languages, involves a simple two-way number contrast between singular and plural (car/cars, child/children, etc.). Discussion of other more elaborate systems of number appears below.
Grammatical number is a morphological category characterized by the expression of quantity through inflection or agreement. As an example, consider the English sentences below:
The number of apples is marked on the noun—"apple" singular number (one item) vs. "apples" plural number (more than one item)—on the demonstrative, "that/those", and on the verb, "is/are". In the second sentence, all this information is redundant, since quantity is already indicated by the numeral "two".
A language has grammatical number when its nouns are subdivided into morphological classes according to the quantity they express, such that:
This is partly the case in English: every noun is either singular or plural (a few forms, such as "fish", can be either, according to context), and at least some modifiers of nouns—namely the demonstratives, the personal pronouns, the articles, and verbs—are inflected to agree with the number of the nouns to which they refer: "this car" and "these cars" are correct, while "*this cars" or "*these car" are ungrammatical and, therefore, incorrect. However adjectives are not inflected, and most verb forms do not distinguish between singular and plural. Only count nouns can be freely used in the singular and in the plural. Mass nouns, like "milk", "silverware", and "wisdom", are normally used in only the singular form. (In some cases, a normally mass noun X may be used as a count noun to collect several distinct kinds of X into an enumerable group; for example, a cheesemaker might speak of goat, sheep, and cow milk as milks.) Many languages distinguish between count nouns and mass nouns.